The subject invention relates to a process for producing easily removable polyimide resin film.
Polyimide resin exhibits excellent mechanical properties, electrical properties and chemical resistance and thus are widely used in multichip modules(MCM), flexible printed circuit (FPC) and tape automatic bonding(TAB).
In the productions of MCM, FPC or TAB, especially multi-clad (multi-layer) boards, it is necessary to form small vias or holes through the clad and then a conductive bridge within the vias or holes to ensure conductive connection across different layers. In the formation of the so-called sprocket holes or device holes, the portion of polyimide film in the position of the hole must be removed.
However, due to its excellent chemical resistance, the etching of fully imidized polyimide is generally difficult. Here imidization means the reaction from the polyamic acid precursor to polyimide final product.
A dry etching process has been used which uses a laser beam to remove polyimide film. Although dry etching process can directly remove polyimide, the initial cost for installing dry etching equipment such as laser etching apparatus is relatively high and the etching rate is very slow.
The alternative of wet etching requires that the polyamic acid (precursor of polyimide) be first partially imidized before etching, followed by fully imidization to polyimide after etching. However, soft baking is required to remove the solvent in the wet etching process and photoresist is often required to selectively etch polyimide.
Among various wet etching methods, conventional chemical etching process for polyimide comprises the application of a photoresist followed by etching with an etching solution. The etching solutions less hazardous to the environment include those disclosed in U.S. Pat. Nos. 4,426,253(1984), 4,431,479(1984), 4,436,583(1984) and 4,848,929 (1989). However, species of polyimide materials which can be efficiently etched by those etching solutions are limited due to the low etching rate. For example, polyimides with trade name Kapton or its precursor with the trade name Pyralin is often used as disclosed in U.S. Pat. Nos. 4,436,583 and 4,848,929.
Furthermore, in the production of adhesive-free TAB, a layer of conductive metallic layer must be coated on a polyimide film. One process is to coat a thin layer of metal by sputtering such as the process of U.S. Pat. No. 4,863,808 in which a metallic film is sputtered on Kapton. However, the cost of this process is relatively high due to the need of the sputtering step.
Another approach for producing adhesive-free TAB is the low cost die-casting method in which a thin layer of polyimide is die-cast on a thin metallic film. However, die-cast TAB suffers from poor etching property and the adhesion and flatness are contradictory to each other. It is therefore very difficult to produce a TAB which has good adhesion and flatness at the same time and which can be easily etched.
Another approach for improving the etching efficiency of polyimide is the use of commercialized photo-sensitive or soluble polyimide material. However, such special material usually exhibit poorer mechanical properties, electrical properties and chemical resistance.